Reduce deprecation warnings in atomic.swift

[ssoper]

No description provided.

[ssoper]

The only remaining warnings (in addition to the anyGenerator) are these

/Users/ssoper/workspace/Safe/Source/chan.swift:100:9: warning: C-style for statement is deprecated and will be removed in a future version of Swift
        for ;; {
        ^
/Users/ssoper/workspace/Safe/Source/select.swift:110:9: warning: C-style for statement is deprecated and will be removed in a future version of Swift
        for ;; {

[tidwall]

I made a note to PR #5 regarding the postfix/prefix operations.

The only concern I have with changing from anyGenerator to AnyGenerator is that it breaks 2.1 and below. Is there a preprocessor conditional for Swift 2.2? Or can we suppress this warning?

[tidwall]

I cleared out all the deprecation warnings except for the anyGenerator and opened a new issue #7.